Hydrothermal Synthesis Of Bismuth-based Photocatalysts And Their Application In The Removal Of Contaminants

In our country,the problem of the water and atmospheric environmental pollution is becoming more and more serious,which causes extensive concern.The power plant emitted into the atmospheric environment about 500-700 tons of mercury per year by coal combustion in our country.Mercury is a kind of poisonous heavy metals,which have higher volatilization and bioaccumulation properties and could cause great harm to the ecological environment and human health.Catalytic oxidation using photocatalytic technology to remove elemental mercury?Hg⁰?emitted from coal-fired flue gas has been recognized by many researchers.The researchers usually adopted TiO₂-based materials as the photocatalysts and the removal efficiency of Hg⁰ are generally low,therefore develop stable and possesses remarkable removal efficiency of Hg⁰ become the hot issues of this research field.The valence band of bismuth based compounds is made up of Bi 6s and O 2p states,which narrow the bandgap and therefore can absorb more light energy.What's more,bismuth based layered material are composed of?Bi₂O₂?²⁺cationic and various kinds of ionic layers,which is beneficial to the transfer of charge between the layers and captured by the water molecules.These properties can effectively suppress the recombination of photogenerated holes and electrons and then improve the photocatalytic activity.Therefore,the layered structures of bismuth compounds possess remarkable photocatalytic performance and have become the research hot issue in the field of photocatalysis.In this dissertation,a series of bismuth-based photocatalysts were fabricated by optimizing the synthesis conditions.The physical and chemical properties of the as-prepared samples were characterized by various techniques.The photocatalytic activities were evaluated by the oxidation removal efficiency of gaseous elemental mercury and the degradation of organic compounds in aqueous under light irradiation.Meanwhile,the relationship between structures and photocatalytic performances was also investigated to research and develop the photocatalysts which were suitable for the oxidation removal of Hg⁰ emitted from flue gas.The main results are listed below:1.BiVO₄ is one kind of bismuth based compounds which can absorb visible light.Mixed crystal structure is in favor of the effective separation of photogenerated hole and electron pair and thus improves the photocatalytic activity of the photocatalysts.In this chapter,the BiVO₄ samples with tetragonal/monoclinic mixed crystal structure were prepared by hydrothermal process using ethylenediamine as chelating agent and alkali source.The influence of ethylenediamine on the morphology and crystal structure of BiVO₄ was studied by controlling the addition amount of ethylenediamine.The as-prepared BiVO₄ photocatalysts were well characterized by a series of measurements.The influence of crystal structure and morphology on the photocatalytic activity was investigated.It was shown that BiVO₄ sample exhibited relatively higher photocatalytic performance for the degradation of Rhodamine B in aqueous under visible-light irradiation.However,the oxidation removal performance of Hg⁰ was very low and the removal efficiency was only ca.15%.This could be ascribed to that the Hg⁰ was mainly oxidized by the holes generated under the light irradiation.The valence band potential of BiVO₄ was relative negative and therefore the oxidation ability of the holes was weak,which could not oxidize Hg⁰ effectively.2.Building composites with heterogeneous structure is a kind of effective method to improve the photocatalytic activity.A series of different proportion of Ag₃PO₄/BiVO₄-x composites were prepared through in-situ deposition method to overcome the low photocatalytic activity for the BiVO₄.The effect of Ag₃PO₄:BiVO₄ratio on the photocatalytic activity was systemically studied.The prepared Ag₃PO₄/BiVO₄-x composites showed excellent photocatalytic activity for the degradation of Rhodamine B and the 2,4-dichlorophenol under visible light irradiation.Compared to BiVO₄,although the photocatalytic oxidation removal efficiency of Hg⁰ was enhanced,the oxidation removal efficiency of Hg⁰ was still low?ca.38%?and not suitable for the removal of Hg⁰ in the flue gas system.3.In order to further synthesis bismuth based photocatalysts,which can effectively photocatalytic oxidation removal of Hg⁰.Meanwhile,the layered structure bismuth based compounds own the internal electrostatic field,which was beneficial to the separation of photogenerated holes and electrons and then improve their photocatalytic activities.In this chapter,a series of Bi₂O₂CO₃ photocatalysts were synthesized by hydrothermal synthesis method.The influences of precursor solution pH value and the carbon source on the crystal structure,morphology and photocatalytic properties of final Bi₂O₂CO₃ products were systematically studied.The photocatalytic performance of the products was evaluated through the removal efficiency of elemental mercury in flue gas.The final products were pure Bi₂O₂CO₃when the pH value of precursor was in the range of 6.65-9.06 using sodium citrate as carbon source,and three products with different dimensional hierarchical morphology were obtained.Carbon source had an important influence on the composition and morphology of the products.Compared with BiVO₄ and Ag₃PO₄/BiVO₄-x composites,the Bi₂O₂CO₃ sample exhibited excellent photocatalytic oxidation activity for the removal of Hg⁰ and the removal efficiency could reach 74%under UV light irradiation 25min.4.BiOIO₃ exhibited superior photocatalytic activity for the degradation of organic pollutants in the aqueous solution because of the heterolayered structure and internal polar field.Therefore,in this chapter,a series of BiOIO₃ photocatalysts were prepared through a simple and mild hydrothermal synthesis method.The influence of precursor solution pH value on the final products was systematically studied.The photocatalytic performance of the products was evaluated through the removal efficiency of elemental mercury in flue gas.Moreover,the stability and the possible mechanism for the Hg⁰ oxidation removal of BiOIO₃ were studied.The generated Hg²⁺after photocatalytic oxidation was also discussed.The results showed that the precursor solution pH value had great influence on composition and morphology of final products.The pure BiOIO₃ products were obtained only under the acidic condition and the pH value of precursor solution did not exceed 3.80.The BiOIO₃sample exhibited superior photocatalytic activity and nearly 100%of Hg⁰ were removed under UV light?25 min?and LED light?100 min?irradiation.The photogenerated holes were the main oxidative species responsible for the oxidation removal of Hg⁰ and almost all of the generated Hg²⁺was flowed into the subsequent liquid absorption system.This new BiOIO₃ photocatalysts with superior photocatalytic activity and good stability have potential application for the Hg⁰oxidation removal.